Hydraulic tunnels and design features

A hydraulic tunnel is an underground conduit excavated through rock or soil to convey water, used in hydropower as headrace, tailrace, diversion or spillway tunnels. It may run free-flow (partly full, like an open channel) or pressure flow (full, under internal pressure).

Design features

1. Alignment: as straight and short as possible; through sound rock, avoiding faults, shear zones and weak strata; crossing weak zones at right angles; adequate rock cover (vertical and lateral) so that the rock weight resists the internal water pressure (Norwegian criterion: cover $C_{RM} \ge \dfrac{h_s \gamma_w F}{\gamma_r \cos\beta}$).
2. Type of flow: pressure tunnels need sound rock and lining for leakage control; free-flow tunnels need about 15–25% freeboard area above the water surface.
3. Shape of section:
   • Circular – best for high internal pressure, minimum perimeter, used for TBM tunnels.
   • Horseshoe / D-shaped (inverted-D) – easy drill-and-blast excavation and flat invert for traffic, common in Nepal.
   • Modified horseshoe – for external rock pressure.
4. Size (diameter): fixed by hydraulic and economic considerations—the economic diameter minimises the sum of construction cost and value of energy lost in friction; minimum size about 2.0–2.5 m for construction access. Usual velocities: 1–2.5 m/s in unlined, 2–4 m/s in lined tunnels.
5. Hydraulic losses: head loss by Manning, $h_f = \dfrac{n^2 V^2 L}{R^{4/3}}$; smooth lining reduces losses ($n \approx 0.012$–0.014 concrete, 0.03–0.04 unlined).
6. Lining and support: unlined in good rock; shotcrete and rock bolts in fair rock; concrete or steel lining in weak rock, low-cover zones and near the powerhouse. Lining reduces friction and leakage and resists water pressure.
7. Grouting and drainage: contact and consolidation grouting; drainage to relieve external water pressure when the tunnel is empty.
8. Slope: gentle slope (about 1:300–1:1000) towards the outlet for drainage and flushing; pressure tunnels kept below the minimum hydraulic grade line everywhere.
9. Transient (water hammer/surge) design: surge tank at the downstream end to limit pressure rise and allow quick load changes.
10. Rock trap and access: a rock trap near the downstream end and adits for construction, inspection and maintenance.

   Circular        Horseshoe        D-shaped
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